Field of the Disclosure
The present disclosure relates to an organic light emitting diode display device, and more particularly, to an organic light emitting diode display device that increases capacitance of a storage capacitor and a method of fabricating the same.
Discussion of the Related Art
Recently, flat panel displays have been widely developed and applied to various fields because of their thin profile, light weight, and low power consumption.
Among the flat panel displays, organic light emitting diode (OLED) display devices, which may be referred to as organic electroluminescent display devices, emit light during loss of electron-hole pairs formed by injecting charges into a light emitting layer between a cathode for injecting electrons and an anode for injecting holes.
The OLED display devices include a flexible substrate such as plastic; because they are self-luminous, the OLED display devices have excellent contrast ratios; the OLED display devices have a response time of several micro seconds, and there are advantages in displaying moving images; the OLED display devices have wide viewing angles and are stable under low temperatures; since the OLED display devices are driven by a low voltage of direct current (DC) 5V to 15V, it is easy to design and manufacture driving circuits; and the manufacturing processes of the OLED display device are simple since only deposition and encapsulation steps are required.
The OLED display devices are classified into a passive matrix type and an active matrix type according to driving methods. Active matrix type display devices have been widely used because of their low power consumption, high definition and large-sized possibility.
FIG. 1 is a circuit diagram of one pixel region of an OLED display device according to the related art.
As shown in FIG. 1, an OLED display device includes a gate line GL, a data line DL, a switching thin film transistor Ts, a driving thin film transistor Td, a storage capacitor Cst and a light emitting diode De. The gate line GL and the data line DL cross each other to define a pixel region P. The switching thin film transistor Ts, the driving thin film transistor Td, the storage capacitor Cst and the light emitting diode De are formed in the pixel region P.
More particularly, a gate electrode of the switching thin film transistor Ts is connected to the gate line GL and a source electrode of the switching thin film transistor Ts is connected to the data line DL. A gate electrode of the driving thin film transistor Td is connected to a drain electrode of the switching thin film transistor Ts, and a source electrode of the driving thin film transistor Td is connected to a high voltage supply VDD. An anode of the light emitting diode De is connected to a drain electrode of the driving thin film transistor Td, and a cathode of the light emitting diode De is connected to a low voltage supply VSS. The storage capacitor Cst is connected to the gate electrode and the drain electrode of the driving thin film transistor Td.
In operation of the OLED display device, when the switching thin film transistor Ts is turned on by a gate signal applied through the gate line GL, a data signal from the data line DL is applied to the gate electrode of the driving thin film transistor Td and an electrode of the storage capacitor Cst through the switching thin film transistor Ts. When the driving thin film transistor Td is turned on by the data signal, an electric current flowing through the light emitting diode De is controlled, thereby displaying an image. The light emitting diode De emits light due to the current supplied through the driving thin film transistor Td from the high voltage supply VDD.
Namely, the amount of the current flowing through the light emitting diode De is proportional to the magnitude of the data signal, and the intensity of light emitted by the light emitting diode De is proportional to the amount of the current flowing through the light emitting diode De. Thus, the pixel regions P show different gray levels depending on the magnitude of the data signal, and as a result, the OLED display device displays an image.
The storage capacitor Cst maintains charges corresponding to the data signal for a frame when the switching thin film transistor Ts is turned off. Accordingly, even if the switching thin film transistor Ts is turned off, the storage capacitor Cst allows the amount of the current flowing through the light emitting diode De to be constant and the gray level shown by the light emitting diode De to be maintained until a next frame.
To do this, capacitance of the storage capacitor Cst needs to be over a predetermined value. However, to implement high definition display devices, the size of the pixel region P decreases, and an area for the storage capacitor Cst also decreases. Therefore, the capacitance of the storage capacitor Cst is lowered. If the area for the storage capacitor Cst is increased, an effective emission area and an area for a compensation circuit are restricted. Accordingly, it is difficult to obtain sufficient capacitance of the storage capacitor Cst.